Configure callback for “Macro apps”

[eugeneia]

This new configure callback (as used in #1068) allows the creation of transparent “macro apps”, which are not instantiated, but instead modify the configuration they are configured on:

— Method myapp:configure configuration, name, arg

Optional. If this method is defined the new method is ignored and not
required, and when myapp is configured using config.app this method is
called instead of instantiating myapp using new. The configure method is
called with the configuration, name, and arg given to config.app, and
is intended to be used to add arbitrary apps and links to configuration using
config.app and config.link.

[wingo]

Excellent idea! The docs should probably note that this is a class method, not a method on an instance, and that it's meant to add to configuration and not have other side effects.